This disclosure is related to the field of pulsed neutron well logging instruments. More specifically, the disclosure relates to methods for operating a pulsed neutron generator and obtaining certain measurements from a plurality of spaced apart, gamma ray spectroscopy detectors so that a plurality of different neutron related parameters of subsurface formations may be obtained while moving the well logging instrument through a selected set of formations only once.
A pulsed neutron well logging instrument can detect/measure neutron-induced gamma rays as function of time or energy. Based mainly on the time spectra data, one can obtain a neutron porosity measurement, a thermal neutron decay time or capture cross section measurement (SIGMA), and an inelastic measurement which is sensitive to gas-filled porosity. Based mainly on the energy spectra data, one can obtain spectroscopy measurements to obtain yields of certain chemical elements in formations surrounding a wellbore. All the measurements based on either time and/or energy spectra can be obtained in the same logging pass. The three neutron measurements based on the time spectra data often have lithology effects, while the spectroscopy can provide an accurate lithology measurement.